It is important to the manufacture of trimellitic acid by the catalytic oxidation of liquid pseudocumene with a source of molecular oxygen (e.g., air) to have cobalt, the most expensive metal oxidation catalyst, recovered and recycled to the liquid phase oxidation. For the liquid phase oxidation of methyl-substituted benzenes in general, it is known that the use of manganese with cobalt as the catalyst metal enhances the production of the benzene carboxylic acid, especially when bromine is the oxidation promoter, more than the sum of the effects of cobalt or manganese alone with bromine. Cobalt has the highest activity of the two metals. Cerium can be used with or to replace part of the cobalt without any loss of the activity due to cobalt especially for the oxidation of liquid pseudocumene to trimellitic acid. But, for such oxidation of pseudocumene to maximize trimellitic acid production and minimize co-production of methylsubstituted phthalic acids (partial oxidation products) as difficultly removable impurities, the manganese component of the Co-Mn-Br or Co-Ce-Mn-Br systems of catalysis is either not added at the start of the oxidation of pseudocumene when all of the cobalt is added or only a portion of the manganese is added at that time. According to U.S. Pat. No. 3,491,144, the addition of manganese is delayed until 50 to 55 percent of the oxidation is complete (i.e., 2 to 2.5 moles of oxygen consumed per mole of pseudocumene) and then manganese is added for its oxidation-enhancing ability. Also, according to U.S. Pat. No. 3,683,016, a portion of the manganese is used initially with cobalt and some of the cerium and the remainder of the manganese and cerium are added to distinct, separately operated oxidation steps, for example, one or two additional steps (a total of two to three steps) operated in series sequence. In such a process the amount of manganese and cerium added to each step is equal to the reciprocal of the total number of steps used.
Thus, to take advantage of the benefits of such staged addition of manganese and still be able to recover and reuse the expensive cobalt component of catalysis for the pseudocumene oxidation, it is important to be able not only to recover cobalt from a residue of the manufacture of trimellitic acid but also to be able to separate cobalt from manganese also so recovered. Such recovery and separation of cobalt and manganese can be accomplished by the inventive method to be next defined and described.